Hundreds of Harry Potter fans are queued patiently, waiting to catch a glimpse of their on-screen heroes at the world premiere of the eighth and final film in the record-breaking cinema franchise.

A small number of hardcore devotees of the boy wizard, created by British author JK Rowling, have been camped out in London's Trafalgar Square since Monday.

The enthusiasm sums up how Rowling's seven novels, and the eight Warner Bros movies based on them, have captured the imagination of millions of fans around the world.

"I have grown up in the Harry Potter generation, it's a limbo state after this," said Rhyss Bowen Jones, 18, who travelled from Wales to queue for a wristband for the premiere of Harry Potter And The Deathly Hallows - Part II.

Lucy Nee, also 18, added: "I can't remember a time when there hasn't been a book or film to look forward to. It's a nice coincidence that Harry finishes school at the same time as me, it feels like we've grown up together."

The new film hits theatres next week, and is the first Harry Potter movie to be released in 3D.

The seven films released so far have earned more than $US6 billion at the global box office, building on the success of the books which have sold more than 400 million copies worldwide.

"As soon as I heard people were setting up camp I rushed down from Kent," said Yasmin, 21.

Some fans were dressed in full wizarding robes and hats, while others read Rowling's novels as they waited for the big night.

Emily Yousaman and Emily Davis, both 16, spent four weeks on their collection of humorous signs, including one reading "Accio Rupert Grint" - a reference to the fictional summoning charm used in the Potter universe.

Actor Grint plays Ron Weasley, one of three principal characters, and like his two co-stars he is a household name and a multi-millionaire thanks to the commercial success of the films.

Daniel Radcliffe plays Harry and Emma Watson Hermione.

All three will be on the red carpet in Trafalgar Square later today to greet screaming fans and give the franchise a star-studded send off.